Compounds are disclosed of formula (I) ##STR1## wherein R.sub.1 represents H or C.sub.1-6 alkyl; R.sub.2 represents H or C.sub.1-6 alkyl; R.sub.3 represents H; R.sub.4 represents H or C.sub.1-3 alkyl; and pharmaceutically acceptable salts and solvates (for example hydrates thereof. The compounds are indicated as useful for the treatment of migraine, cluster headache, chronic paroxysmal hemicrania and headache associated with vascular disorders. Processes and intermediates for their preparation and pharmaceutical compositions containing them are also disclosed.